There exist various methods and apparatus for coating a photographic support. One such coating system uses a bead coating hopper. Another system employs a curtain coating hopper. These systems are used in coating the surface of a support with single or multiple coatings, the latter of which can be coated simultaneously onto the support. The fabrication of photographic light-sensitive elements typically requires the simultaneous coating of multiple emulsions layered on the support.
The coating apparatus, and in particular the coating hopper, can be fabricated from various materials, the selection of which depends largely on the coating environment. To avoid the introduction of contaminants into the product and to prolong the working life of the apparatus, photographic emulsion-coating apparatus is typically fabricated from materials such as AISI 304 or 316 stainless steel which possess good resistance to corrosion and wear. Stainless steel, however, has the drawback of also possessing a high coefficient of thermal expansion, which at high coating temperatures such as at 50 to 100 degrees C. can result in non uniform coatings due to dimensional nonuniformities in the coating hopper caused by uneven thermal expansion, especially with multiple coating hoppers. Multiple hoppers comprise multiple hopper bars stacked one on another, each bar typically about 5 feet long, 6 inches wide, and 2 inches thick and having an intake slot and a discharge slot. The size and configuration of the slot opening tends to fluctuate appreciably due to changes in the emulsion coating temperature and due to bending in the coating bars caused by their weight. Also, streaks and other imperfections can be introduced into the coated layer or layers. The streaks and coating nonuniformities are often considered minor and acceptable in many non photographic coating applications. Even minor such defects, however, can be unacceptable in photographic applications, because they can lead to optical distortion in a developed photographic image that becomes even more apparent and significant upon enlargements or in products having to meet high quality standards, such as professional photographic film and paper products and motion picture film and those for graphic art.
EPA 0361167 discloses a coating apparatus fabricated from a sintered or hot-pressed ceramic material having a higher modulus of elasticity and a lower coefficient of thermal expansion than a metal such as stainless steel. A drawback, however, is that sintered and hot-pressed ceramics are porous and have other surface irregularities, which will be collectively referred to herein as "voids". Voids provide sites upon which a coating liquid can accumulate and interfere with the smooth, continuous flow of the coating liquid, thereby introducing nonuniformities and streaks into the coating layer or layers. Unlike stainless steel, the porosity of such ceramics can cause voids on the coating hopper lip where the coating or coatings flow from the hopper to the photographic support which also introduces streaks and other defects into the coating layers. Another problem caused by the presence of voids is that solidification of accumulation coating liquid in the voids causes difficulty in cleaning the coating apparatus, resulting in more out-of-service time of the equipment.
To minimize or eliminate voids, U.S. patent application Ser. No. 08/068,468 filed May 27, 1993, now U.S. Pat. 5,484,629, discloses surface treatment such as chemical vapor deposition (CVD) coating followed by grinding and lapping or a densification process hot isostatic pressing followed by grinding and lapping using hopper bars having a ceramic coating surface. Voids provide sites upon which a coating liquid can accumulate and interfere with the smooth, continuous flow of the coating liquid, thereby introducing non uniformities and streaks into the coating layer or layers.
Although ceramics, such as silicon carbide, SiC and silicon nitride, Si.sub.3 N.sub.4, are more ideal emulsion coating hopper materials than metals, it is not always feasible to fabricate production size multiple bar hoppers from the standpoint of economics and having available commercial facilities large enough to accommodate such fabrication. Also from the engineering viewpoint, it is not easy to assemble a monolithic ceramic hopper using metal fasteners. Materials handling during manufacture of the hopper bars and transportation to the production site are major obstacles to produce monolithic ceramic emulsion coating hoppers. Furthermore, threaded holes needed for plumbing are not feasible in brittle ceramics. It is therefore an object of this invention is to fabricate a photographic emulsion coating apparatus and method for producing photographic emulsion coatings on polymeric support that are uniform and substantially free of streaks and are economical.